A Novel Method for Remotely Detecting Trace Explosives · A Novel Method for Remotely Detecting Trace Explosives Charles M. Wynn, Stephen Palmacci, Roderick R. Kunz, and Mordechai

A Novel Method for Remotely Detecting Trace ExplosivesCharles M. Wynn, Stephen Palmacci, Roderick R. Kunz, and Mordechai Rothschild

Events in recent years have led to an

increased need for improvements in our

abilities to detect explosives. In numerous

situations, the capability to detect explosives

sensitively, accurately, and rapidly could have great benefit

to national security both at home and abroad. Some of the

homeland situations that would benefit from improve-

ments in explosives detection include screening passen-

gers and luggage at airports and other sensitive locations,

and screening vehicles and people along the perimeters of

high-value installations such as federal buildings. Abroad,

the improvised explosive device (IED) problem is clearly

in need of creative solutions to mitigate the devices’ very

damaging effects. Much research has been conducted to

help solve these problems; however, it is unlikely that any

one solution will suffice.

If we view the problem of explosive devices as a

timeline, from the initial planning to construction of the

devices and ultimately to their detonation, we can see

that it is preferable to detect the activities as early in the

process as possible. The term “left of boom” derives from

displaying the explosives creation-to-detonation timeline

from left to right and is sometimes used to refer to the

concept of early explosives detection. Figure 1 shows a

graphical representation of this process.

To address these concerns, we have begun several

efforts to both understand and help solve the difficult prob-

lem of explosives detection. In particular, we are focus-

ing on a solution for rapid and remote detection of trace

amounts of explosives, with the idea that such a capability

would significantly improve our ability to respond left of

boom. As we discuss later, bomb making is a messy process

The development of a technique with the ability to detect trace quantities of explosives at a distance is of critical importance. In numerous situations when explosive devices are prepared, transported, or otherwise handled, quantifiable amounts of the explosive material end up on surfaces. Rapid detection of these chemical residues in a noninvasive standoff manner would serve as an indicator for attempts at concealed assembly or transport of explosive materials and devices. We are investigating the use of a fluorescence-based technique to achieve the necessary detection sensitivity.

»

A NOVEL METHOD FOR REMOTELY DETECTING TRACE EXPLOSIVES

that leaves behind quantifiable residues on a variety of sur-

faces. Trace-detection techniques could thus be used foren-

sically, identifying devices and bomb-related activities,

such as assembly, earlier in the process and thus prevent-

ing the need for more difficult countermeasures later on.

To be practically useful, the technique must be rapid and

sensitive. Additionally, long-range capabilities that could

scan from distances of at least ten meters open up numer-

ous possibilities in which large areas of interest could be

rapidly assessed for indications of explosives activity. For

a brief explanation of the issues we face in trace detection

of various explosives, see the sidebar “Explosives and Their

Characteristics” on page 30.

We have been investigating a multistep technique in

which the first step is a dissociation of polyatomic materi-

als into diatomic molecules. The technique is known as

photodissociation followed by laser-induced fluorescence

(PD-LIF). The laser powers required for this technique are

significantly lower than those of some other methods, and

there is a good possibility of successful detection by using

eye-safe lasers. When nitro-bearing explosives are illumi-

nated with ultraviolet (UV) light within their absorption

band (see Figure 2) and of a sufficient intensity, the mol-

ecules dissociate (the PD step of PD-LIF), creating, among

other things, fragments of nitric oxide (NO). Identification

of this NO photofragment through laser-induced fluores-

cence (LIF) forms the basis of our detection technique. NO

(not to be confused with nitrous oxide, N2O) can be found

in smog. There is, however, a very important difference

between the NO produced in the dissociation of explosives

and that which can occur as a pollutant. The atmospheric

pollutant exists in its ground state, while the NO produced

in the photodissociation process is initially produced with

excess vibrational energy. This excess energy is extremely

significant in that it not only allows us to distinguish

explosives from smog, but also, as we show later, allows

us to devise a detection scheme that should be relatively

immune to many common types of optical clutter (see the

sidebar “Optical Clutter” on page 34).

We have performed extensive studies [1] of the

bomb-making process to determine the sensitivity levels

necessary for detecting microscopic explosives residues.

When explosive devices are prepared, transported, or

otherwise handled, quantifiable amounts of the explosive

material end up on both people and related surfaces. The

primary vectors of transport are the hands and feet of the

bomb makers and bomb handlers. Even when people do

not directly contact explosive material, they may trans-

fer quantifiable amounts of residue that they pick up via

secondary transfer. Such a transfer occurs when some-

one directly handling explosives touches a surface such

as a countertop or door handle, and then someone else

touches that surface. The second person will pick up a

quantifiable amount of explosives upon contact with the

surface and may then transfer it to additional surfaces.

We carefully quantified the amounts that were trans-

ferred to surfaces under a variety of conditions. These

surfaces, such as vehicles, luggage, clothing, walkways,

and doors, could potentially be used as early indicators

of activity. Although we identified large variability in the

amount of explosives residues transferred to surfaces,

in most instances, a majority of surfaces of interest had

quantities of at least 1 μg/cm2. We use this quantity as

a rough estimate of the required sensitivity for a foren-

FIGURE 1. The interest in detection of explosives is focused on the early timeline activities, including the planning stages, although after-action post-detonation evaluation could determine the detonation products for forensic pur-poses. After-action activities on the part of the perpetrators would include evaluating the effects of the explosion. CONOPS stands for concept of operations.

Obtainfunds

Developorganization

Gather andprovidematerials

Planattack

Performattacks

ImproviseCONOPS,tactics,devices

Evaluateafter-actionactivities

BOOM

CHARLES M. WYNN, STEPHEN PALMACCI, RODERICK R. KUNz, AND MORDECHAI ROTHSCHILD

sic technique. Note that this areal coverage amounts, on

average, to only a few monolayers of explosives.

Trace-Detection TechniquesGiven the characteristics detailed in the sidebar on

explosives, what sorts of techniques are possible, if we

keep in mind the constraint that we require something

rapid, sensitive, and remote? A number of techniques

are currently in use for point detection (at a location),

including ion-mobility spectrometry (in use in many

airports), gas-chromatography mass spectrometry (gen-

erally a laboratory technique), and a somewhat newer

technique based on the fluorescence quenching of poly-

mers. The detector employing the latter is known as

Fido. The Fido explosives detector is built by Nomadics,

Inc. (a business unit of ICx Technologies, Inc.) and is

based on a proprietary technology developed by Timothy

Swager at MIT. The product is so named because its level

of detection is comparable to that of a highly trained

explosives-detection dog. All these techniques can detect

minute quantities of explosives (generally in the pg-to-

ng range). However, they all share one common property

that makes them ill suited for standoff detection. They

all require ingestion of the explosives into their detec-

tors. In other words, they must come into direct physical

contact with the explosive molecules themselves. Given

the vanishingly low vapor pressures of explosives (Fig-

ure 2), it does not appear feasible for us to operate these

techniques remotely.

We found that, in general, a useful detection tech-

nique must fulfill two potentially conflicting require-

ments. First, it must have high sensitivity: it must be able

to detect a signal generated from as little as a single mono-

layer of material. For the technique to operate remotely,

this signal must be accessible from several meters’ dis-

tance. None of the above techniques appears to us to be

able to meet this distance requirement. Second, the tech-

nique must have high specificity. The signal generated

by explosives should be readily distinguishable from the

signal generated by other common materials. We use the

term chemical clutter to refer to nonexplosive materials

with optical characteristics similar to those of explosives.

In our case, these materials might include other NO-con-

taining compounds. False alarms may also occur as the

result of optical clutter (see the sidebar “Optical Clutter”).

We use the term optical clutter to refer to photons that are

generated by a source other than explosives and that do

not share the same optical characteristics as our signal.

While optical clutter can be addressed via optical filtering

techniques, chemical clutter could pose a more serious

difficulty. Additional considerations for a successful tech-

nique include its practicality—the system should have a

minimum of components and complexity to facilitate ease

of use in a real-world situation.

200

0

20

40

300 400 500 600200Wavelength (nm)

Groundstate

Vibrationallyexcited

2700 KC

ross

sect

ion

(10

–18 c

m2 /m

olec

ule)

210 220 230 240 250 260Wavelength (nm)

Abso

rptio

n (a

rbitr

ary

units

)

60

80

100

TNT

= N = O = C

NO

FIGURE 2. The analysis of explosives is improved with the detection and distinction of NO units. These optical absorp-tion spectra of molecular species show the improvement. The top graph is of TNT, a polyatomic molecule in the solid phase. Note that the optical absorption spectra of other organic nitro-bearing explosives are generally similar to that of TNT because they have a broad peak in the deep ultravio-let (UV). The bottom graph is of nitric oxide (NO), a diatomic molecule in its gas phase. Both the absorption spectra of the ground (solid lines) and the first vibrationally excited (dashed lines) states are shown. Photodissociation, the first step of the photodissociation laser-induced fluorescence (PD-LIF) process, creates the distinct NO spectrum from the generally featureless spectrum of TNT. Also note that PD-LIF creates NO in an excited state, which allows for a low-clutter detection technique to be applied.


Explosives may be broken down into two general classes: nitro/nitrate-based and non-nitro/nitrate-based. Non-nitro/nitrate-based explosives are derived from materials such as peroxides, e.g., triacetone triperoxide (TATP), per-chlorates, and azides. While these explosives clearly pose a threat, our studies focused upon the more common nitro-based explosives. The military primarily uses these types of explosives; they include compounds such as nitroglycerin, 2,4,6-trinitrotoluene (TNT), and hexahydro-1,3,5-trinitro-1,3,5-triaz-ine (RDX), the active component

in the plastic explosive C4. These materials make up a large percent-age of the explosives used in impro-vised explosive devices.

To develop and/or assess the utility of a trace-detection tech-nique, we have to understand some of the physical characteristics of these explosives. Figure A shows the chemical structure and vapor pressure of some of the more com-mon nitro-based explosives. Other key characteristics of explosives are the following:• Explosives have extremely low vapor pressures, ranging from parts per million to less than parts

per quadrillion, in the case of octa-hydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX). Low vapor pressure places severe limitation on a technique that relies on the ambi-ent vapor above the material for detection. The solid residue itself holds much more potential signal than the vapor phase. • Nitro-based explosives all have at least one NO2 group. When nitro-based explosives detonate, one of the ultimate end products is N2 gas, which is very stable due to its N-N triple bond. The formation of this low-energy product from the higher-energy starting material is

Explosives and Their CharacteristicsSome compounds are easier to detect than others.

We concluded that optical techniques appear to have

the greatest potential for remote-detection capability.

A variety of approaches to optical detection have been

attempted [2], and we found it instructive to examine them

in the context of the above-mentioned constraints. UV illu-

mination of explosives induces a relatively strong fluores-

cence signal. However, it is not very specific to explosives,

since many materials fluoresce under UV illumination and

their fluorescence spectrum is often nonspecific. Thus this

technique does not fulfill the specificity constraint; optical

clutter would be a significant problem. Other techniques

generate a reflection signal based on the absorption peaks

in explosives. Again, optical clutter appears to be a prob-

lem because the spectra are broad and not necessarily

unique to explosives. Furthermore, only a very thin layer

of material (monolayer) is being interrogated; thus it will

not absorb much light beyond the ultraviolet. Additionally,

and perhaps more importantly, the explosives distribution

is nonuniform, such that much of the reflected signal may

not be dominated by the explosives themselves but by the

surface upon which they reside.

A more promising technique utilizes Raman spec-

troscopy. It has been known for some time that there is a

weak but distinct interaction between photons illuminat-

ing a material and the vibrational modes (phonons) of the

material. The result of this interaction is a scattered pho-

ton with either slightly more or less energy than that of

the incident photon, depending upon whether a phonon

has been absorbed or created. This inelastic scattering

process creates a spectral fingerprint of the material spe-

cific to its vibrational structure; thus it is expected to have

reasonably low clutter. While these Raman energy shifts

are successfully used to identify materials at very close

range in the laboratory, the scattered signal is so weak that

detection at long range would require the combination of

large collection optics and long integration times.

An alternate approach, which removes many of the

difficulties associated with the indistinct spectra of the

materials themselves, is to dissociate the materials and

use instead signals related to their constituents. Laser-

induced breakdown spectroscopy (LIBS) is one such tech-

nique, which in its most common incarnation analyzes


accompanied by the release of a sizable amount of energy. Note that inorganic nitrate-based explo-sives have NO3 groups instead of NO2 groups (ammonium-nitrate fuel oil is one such example). Our initial work focused on organic materials that contain NO2 groups.• The explosives residues relating to bomb activities can be, on aver-age, only a few monolayers thick. However, they are nonuniform, taking the form of mounds or clumps at the microscopic level. This nonuniformity means that some percentage of a surface of interest may be bare, containing no explosive material at all.• The optical absorption spec-trum (Figure 2 in the main text) of nitro-bearing explosives is

broad and nondistinct. There is a broad absorption peak in the ultra-violet (UV), and the explosives will fluoresce in the visible as a result of UV illumination.

REFERENCES

a. National Research Council, Existing and Potential Standoff Explosives Detection Techniques (The National Academies Press, Washington, D.C., 2004).

b. J. Steinfeld and J. Wormhoudt, “Explosives Detection: A Challenge for Physical Chemistry,” Ann. Rev. Phys. Chem., vol. 49, 1998, pp. 203–232.

FIGURE A. The vapor pressures of some nitro-bearing explosives are quite low and span orders of magnitude, indicating the dif-ficulties of trace detection of vapors [a, b]. TNT is 2,4,6-trinitrotoluene, RDX is hexa-hydro-1,3,5-trinitro-1,3,5-triazine, PETN is pentaerythritol tetranitrate, and HMX is octa-hydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocane.

10–12

10–15

10–9

10–6

Rela

tive

conc

entra

tion

in a

ir

Nitroglycerin

TNT

PETNRDX (C4)

HMX

Common feature:NO2

= N = O = C

the atomic constituents. LIBS uses tightly focused, high-

energy laser pulses to dissociate the materials into their

constituent atoms via the formation of a microplasma.

The atomic emission spectra allow identification of the

atoms and estimation of their relative abundances. A

nitrogen/oxygen ratio consistent with explosives is used

as an indicator of the presence of a specific explosive.

While the LIBS signal is strong, the relative ratios can be

easily obscured by the existence of other nitrogen and/or

oxygen-bearing materials present on surfaces. In other

words, LIBS is highly susceptible to clutter [3, 4]. In

addition, there is the disadvantage of using high-intensity

lasers that aren’t eye safe.

Photodissociation Followed by Laser-Induced FluorescencePD-LIF is a dissociation-based technique in which the

polyatomic materials are dissociated into diatomic mol-

ecules, as opposed to the atoms produced via LIBS. The

laser powers required for the PD-LIF process are sig-

nificantly lower than those of LIBS, and there is a good

possibility (as we discuss later) that successful detection

can occur if eye-safe lasers are used (again in contrast to

LIBS). When nitro-bearing explosives are illuminated

with UV light within their absorption band (see Figure

2) and of a sufficient intensity, the molecules dissociate

and create, among other things, fragments of NO. Identi-

fication of this NO photofragment forms the basis of our

detection technique. As mentioned earlier, the NO prod-

uct of PD has excess vibrational energy when compared

to the potential chemical-clutter components of NO in

smog, for example.

An additional advantage of having produced the NO

molecule via photodissociation is that we are now deal-

ing with a gas-phase diatomic molecule (as compared to

a solid-phase polyatomic molecule), which has a much

more distinct spectrum that can be used as an identify-

ing fingerprint. The lower portion of Figure 2 displays

the absorption spectrum of NO. Note the distinct lines, as

compared to the broad spectrum of 2,4,6-trinitrotoluene

(TNT). Also note the different spectra for the ground and

excited states of the molecule. As can be seen from the


figure, the energy difference between the two is ~2700 K:

thus the excited state is not occupied under standard

ambient temperatures. In PD-LIF, detection of the vibra-

tionally excited NO is achieved via laser-induced fluores-

cence. Here, a photon further pumps the vibrationally

excited NO fragment into an electronically excited state.

The electronically excited NO molecules rapidly fluoresce

as they return to their ground states. Because of the initial

excess of vibrational energy, this fluorescence occurs at a

shorter wavelength (226 nm)—i.e., higher energy—than

the exciting laser photons (236.2 nm). Signal photons that

are of shorter wavelengths, or shifted toward the blue end

of the spectrum relative to the laser photons, are critical

to minimizing optical clutter. A schematic of this multi-

step (photodissociation/vaporization—photoexcitation—

fluorescence) detection process is shown in Figure 3. In

brief the steps are the following:

1. The first photon is absorbed by the explosives

and very rapidly (<<1 ns) vaporizes and dissoci-

ates the explosives’ components into fragments

including the vibrationally excited NO.

2. A second photon pumps the vibrationally

excited NO to an electronically excited state.

This pumping must happen within a few ns,

which is the lifetime of the vibrationally excited

state in a standard atmosphere.

3. The resulting NO fluorescence, which occurs

at a shorter wavelength than that of the laser, is

used for detection.

Note that all these steps occur within a single laser

pulse. The exquisite specificity of this technique derives

from the precise wavelengths involved in the LIF process.

It is highly unlikely that many other materials will pro-

duce photons at precisely 226 nm in response to illumi-

nation at precisely 236 nm, since few will share the exact

spectral structure of the excited NO. Thus we can utilize

a narrow-wavelength source and a narrowband detec-

tor to specifically detect these NO fragments with a high

degree of sensitivity and specificity. We note also that it is

fortuitous that the nitro-bearing explosives absorb quite

well at the UV wavelength needed to pump NO. Because

of this absorption, we can accomplish the multiphoton

process with a single laser pulse, significantly simplifying

the necessary equipment.

Advantages of this detection method include a rela-

tively strong fluorescence signal—which can be gener-

ated by using eye-safe excitation-laser intensities—and a

low false-alarm rate. The low false-alarm rate is expected

because relatively few processes produce light at shorter

wavelengths than the source laser. In contrast, most other

Pulsed UV laser2

1

2

226 nmVibrationallyexcited

nitric oxide Internuclear distance

S 2Σ

X 2Π

Explosives residue(nitro-bearing)

Electronicstates

Ener

gy Vibr

atio

nal

leve

ls

33

236.2 nm

= N = O = C

1. Vaporize andphotodissociate

solid explosive �� h�� excited NO vapor

3. Detect higher-energyphotons

Eliminates red-shiftedfluorescence clutter

Photo-detector

2. Optically pump NO(same laser pulse as step 1)

FIGURE 3. The PD-LIF explosives-detection technique can be understood as a three-step process. In the first step, a mate-rial with a distinct signature is created by vaporizing and dissociating solid explosives and forming gaseous NO fragments. In the second step, these fragments are optically pumped to a higher-energy state. Finally, the higher-energy fluorescence emis-sion is detected. The wavelength of the fluorescence and the wavelength required to induce fluorescence are both very pre-cise, which provides the specificity of the technique.


processes, such as scattering or traditional fluorescence,

are single-photon processes in which each incident pho-

ton yields another of equal or lesser energy. The PD-LIF

process involves multiple photons, the first of which

creates a molecule with excess energy and the second

of which further excites that molecule. As the molecule

relaxes to its ground state (see Figure 3), photons are

emitted with greater energy than that of the incident

photons. As such, this technique isn’t susceptible to false

alarms from traditional fluorescence processes, for which

the resultant photons typically are at longer wavelengths.

The sidebar on optical clutter shows how these higher-

energy emitted photons are distinguishable from typi-

cal fluorescence coming from the surfaces on which the

explosives reside.

PD-LIF Detection MeasurementsPrevious studies by others [5] have examined the PD-LIF

signature from explosives vapors. We, however, probed the

solid and liquid explosives themselves [6, 7] as they offer

the potential for much greater signal, since the vapor pres-

sure of explosives is very low. The four main components

in single-laser PD-LIF laboratory experiments shown in

Figure 4 are the excitation laser, the photodetector, the

bandpass optical filters, and the samples being tested. In

our studies, the laser was a tunable pulsed Continuum

9030 system, which provides variable laser output from

215 to 310 nm. The laser output was an ~1 cm2 beam of

7 ns pulses, emitted at a rate of 30 pulses per second, with

energies of 2 to 3 mJ per pulse. The spectral linewidth was

0.03 to 0.04 nm around the wavelengths of interest. All

wavelengths reported are the values in air (approximately

0.03% less than the wavelengths in a vacuum).

The fluorescence detection subsystem consisted of

a solar-blind (tuned to eliminate the background solar

radiation) photomultiplier tube (PMT) with narrowband

filters. Narrowband filters are essential in these experi-

ments because they are needed to suppress the scattered

laser light. The Perkin Elmer cesium-tellurium channel

photomultiplier had single-photon sensitivity, negligible

dark counts, and a quantum efficiency of 10% near the

signals of interest (220 to 230 nm). The PMT was used

in both a photon-counting and linear mode (dependent

upon photon flux) and was calibrated for short laser

pulses at various cathode voltages in order to ensure a

linear response at a given flux level. Experiments were

performed primarily in a close-range geometry, so that

the PMT and filters were positioned 6 cm directly above

the sample. No collection or collimation optics were used

in the close-range configuration. In this geometry, we

estimate a collection efficiency of 4 × 10–7 of all emitted

signal photons.

A variety of explosives in a variety of morphologies

were studied. The explosives included 2,6-dinitrotoluene

(DNT), TNT, pentaerythritol tetranitrate (PETN), and

hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). DNT, as

received from Sigma-Aldrich, was in the form of small

granules roughly the size of salt crystals. Studies were

performed on macroscopic mounds (several milligrams)

Pulsed UV laser

Photodetector

Optical filter

TNT residueon sand

FIGURE 4. The four primary components of the experi-mental detection setup are shown: the pulsed UV laser, the high-rejection optical filters (which are normally attached directly to the photodetector and allow only signal photons to reach the detector while blocking the scattered laser pho-tons), the photomultiplier tube photodetector, and the sam-ple being tested (here it is sand covered with traces of TNT).


of these solid granules, in addition to a liquid form, which

was obtained by heating the granules to 80°C. With the

exception of the liquid DNT measurements, all other

measurements were performed at room temperature

under ambient atmospheric conditions. Military-grade

TNT was studied in a solid pellet form, as a trace coat-

ing on sand, and dropcast from a dilute acetone solution.

Military-grade PETN was studied in the form of a white

powder. RDX was studied both as a trace coating on sand

(8% by weight) and as the dominant component (~90%)

in the putty-like C4 plastic explosive.

Figure 5 displays the results of fluorescence detec-

tion measurements of DNT, TNT, C4 (RDX is the active

component), and PETN. Data were taken at a fluence

of 10 mJ/cm2/pulse (1-mJ pulses over 0.1-cm2 area).

Data points represent 6-pulse averages, with the excep-

tion of the background measurements on the bare silica

substrate (shown as the open squares in the top graph)

for which the data points are 60-pulse averages. All

Detection of spurious radia-tion, or optical clutter, causes an increase in false-alarm rates. Potential sources of optical clut-ter can be either ambient or laser-induced radiation, as shown in Figure A.

Ambient Clutter. While solar radiation is a dominant source of clutter in the visible and near infra-red, and thermal radiation can be problematical in the infrared, the deep ultraviolet (UV) is quite clutter free. Below approximately 300 nm (the solar-blind region), the absorp-tion due to atmospheric ozone pre-vents almost any light from reaching the surface of the earth [a]. This

fact opens up the possibility of operation of a detection system that relies upon a very small num-ber of signal photons (perhaps as few as one) for detection. The PD-LIF detection system we are pursuing operates in the deep UV and thus aims to utilize just such single photons.

Laser-Induced Clutter. Laser-induced clutter can be sub-divided into three broad spectral categories: laser scatter, i.e., pho-tons at the laser wavelength; red-shifted clutter (photons with less energy and longer wavelengths than the laser); and blue-shifted clutter (photons with more energy

and shorter wavelengths than the laser). Generally, the scattered photons (at the laser wavelength) are the strongest source of clutter. Fluorescence processes (the domi-nant form of red-shifted clutter) can yield signals as strong as ~10% of the laser scatter, depending on the material and wavelengths chosen. Note that many materials fluoresce when irradiated with UV light (just visit your local roller rink). Photons that are created with more energy than the energy of incident photons are quite rare because they require the addition of excess energy. Thus blue-shifted clutter is much weaker than in either of the other two

Optical ClutterReduce the effects of clutter on false-alarm rates by taking measurements where spurious fluorescence isn’t.

explosives samples display the same multipeak struc-

ture with a maximum signal at the excitation wavelength

of 236.2 nm and a shoulder near 236.3 nm. They also

display a secondary peak at 236.9 nm, again with an

accompanying shoulder.

The similarity of these signatures implies that all

nitro-bearing explosives can be detected via this tech-

nique without the need to fine-tune the laser parameters

to each individual explosive type. The vapor pressures

for these compounds differ significantly—by over four

orders of magnitude at room temperature—while their

signal strengths are within one order of magnitude. These

measurements indicate that the observed signal is not

related to the ambient vapors of the materials, but rather

the condensed phase itself. This is an important point in

our consideration of solid-phase analysis: a technique

that relies upon detection of the ambient vapors of the

explosives will be severely limited due to their very low

vapor pressures [8, 9].


236.2 nm, complete photolysis of TNT takes about 10

laser pulses, as shown on the right in the figure. On the

basis of these results, we can estimate that in our specific

experimental configuration we were detecting 20 ng TNT

with each laser pulse (2 μg/cm2 times 0.1 cm2/10 pulses).

Also displayed in Figure 6 is the photo response from a

control sample (the bare silicon wafer), which is signifi-

cantly weaker than that of the TNT, indicating a relatively

high signal-to-noise ratio for detection.

Remote Detection ProjectionsOur trace-detection results along with the spectra of the

different explosives indicate a technique with the required

sensitivity to detect the trace amounts of interest. Keeping

in mind the eventual application of PD-LIF to the stand-

off detection of these residues, we need to estimate the

detection range of a notional system. For a first step, we

calculate the relative efficiency of this process, compared

to the alternative of Raman spectroscopy. (While compar-

regions. Anti-Stokes Raman scat-tering involves excess energy that comes from the vibrational energy (phonons) of the material scattering the light. Processes that involve the simultaneous absorption of more than one photon (multiphoton pro-cesses such as our PD-LIF) can also produce a blue-shifted signal. The cross sections for all these pro-cesses are exceedingly weak, and thus this regime offers a useful por-tion of the spectrum in which to do low-clutter detection.

REFERENCE

a. A. Thompson, E.A. Early, J. DeLu-isi, et al., “The 1994 North Ameri-can Interagency Intercomparison of Ultraviolet Monitoring Spectroradi-ometers,” J. Res. Natl. Inst. Stand. Technol., vol. 102, no. 3, 1997, pp. 279–322.

FIGURE A. Ambient clutter is present whenever the system is in an environ-ment where sources other than the laser are present. Such clutter is shown In the upper image, specifically for solar-induced scatter. The laser-induced clut-ter exists in three wavelength regions. The laser scatter (at the laser’s wave-length) and the lower-energy red-shifted scatter do not affect the PD-LIF measurement, which occurs at a higher-energy blue-shifted wavelength.

Solar spectrum

Solar illumination (daytime) is 1015–1017 photons/s/cm2

Sola

r flu

x(p

hoto

ns/s

/cm

2 /nm

)

100

10–4

108

104

1012

Wavelength (nm)200 1200800600 1000400

Solar-blindregion

Upper-bound flux(MIT LL measurements)

Rela

tive

signa

l

Optical clutter

100

10–15

10–910–6

10–12

10–3

Blue-shifted clutterAnti-Stokes, multiphoton

Lase

r sca

tter

More energy than laser Less energy than laserλ

Laser Red-shifted clutterTraditional fluorescence

The bottom graph of Figure 5 displays the predicted

fluorescence spectrum of NO, assuming the PD-LIF exci-

tation (excitation from the first vibrationally excited state

of its electronic ground state to the vibrational ground

state of its first electronically excited state). We obtained

these results by using the LIFBASE software package

[10, 11]. Comparison of the experimental data to the

computed NO spectrum provides clear evidence that the

measured signal is generated by excited NO species.

We used calibrated quantities of explosives dissolved

in acetone to demonstrate trace-level detection. These

explosives were dropcast on silicon wafers to yield areal

concentrations of 2 μg/cm2 (shown in Figure 6), similar

to concentrations expected from fingerprint residues. We

investigated both RDX and TNT in this manner and mea-

sured signals roughly half of those measured for the bulk

materials. Laser-induced photolysis of the trace material

is evident in the top portion of the left-hand figure. For

laser fluences of 10 mJ/cm2/pulse and a wavelength of


ison to LIBS is also possible, PD-LIF and Raman-based

detection have in principle similar low false-alarm rates.)

The Raman cross section for TNT has been reported as

2 × 10–31 cm2/sr/molecule for visible excitation and 10–28

cm2/sr/molecule for ultraviolet resonant excitation, and

therefore the angle-integrated values are 2.5 × 10–30 cm2/

molecule and 10–27 cm2/molecule, respectively [12, 13].

The spontaneous Raman scattering process is linear in

fluence, and therefore the cross section is a constant. In

contrast, the PD-LIF is nonlinear in fluence because it is

a multiphoton process. A cross section will therefore be

fluence dependent. Furthermore, it will be an effective

parameter rather than a true molecular property.

By analogy to a linear process such as Raman scatter-

ing, the number of fluorescence photons collected in our

experimental setup, Nphoton, can be expressed as follows:

N Fnphoton eff= σ η ,

where F is the laser fluence (in photons per unit area), n is

the number of molecules being irradiated, σeff is the effec-

tive cross section of the overall PD-LIF process, and η is

the experimental collection efficiency. We have estimated

the PD-LIF cross section, σeff, on the basis of our dropcast

results at a fluence F = 10 mJ/cm2/pulse (1.25 × 1016 pho-

tons/cm2). At this fluence, we collect Nphoton ~10 photons/

pulse. We can estimate the number of irradiated mole-

cules, n, by using the areal density of the dropcast TNT

(2 μg/cm2), the laser spot size (0.1 cm2), and the molecu-

lar weight of TNT (227 g/mole).

Given that ~10 laser pulses were required to fully pho-

tolyze the film, and that the total number of molecules in

the film was 5 × 1014, we estimate that n = 5 × 1013 mol-

ecules/pulse were irradiated. Note that the average thick-

ness of the dropcast TNT is ~12 nm (the density of TNT is

1.65 g/cm3), similar to our measured photolysis rate per

pulse. However, we observed experimentally that 10 pulses

were required to photolyze the film, rather than one pulse,

as may be indicated by the average film thickness. These

results are reconciled by noting, as is evident from optical

images of the films, that the films are not uniform. Rather,

they consist of islands of different sizes and, presumably,

thicknesses. Using our collection efficiency of η = 4 × 10–7,

we calculate σeff = 4 × 10–23 cm2/molecule for TNT at

10 mJ/cm2/pulse fluence. This is a factor of 4 × 104 higher

than that achievable with Raman scattering in the opti-

0.0

0.5

1

2

0

4

8

12

0

1.0

235

NOfluorescence

PETN (solid)

C4 (solid)

TNT (solid)

DNT (liquid)

236 237 238Pump laser wavelength (nm)

Sign

al (a

rbitr

ary u

nits)

Sign

al at

226

nm

(p

hoto

ns/p

ulse)

4

8

12

0

0

5

10

15

20

25

Sign

al at

226

nm

(p

hoto

ns/p

ulse)

Sign

al at

226

nm

(p

hoto

ns/p

ulse)

Sign

al at

226

nm

(p

hoto

ns/p

ulse)

FIGURE 5. The response of NO-based materials to pump laser wavelength is selective. The shifted peaks and shoul-ders show a marked response to the NO first excited state. The top graph also includes a background measurement of the response of the silica substrate without explosives (open squares). The bottom graph is the predicted fluorescence of NO, assuming excitation from its first vibrationally excited state. Note that the emission from these materials is actu-ally at the higher-energy blue-shifted 226 nm wavelength.


mal condition of UV excitation, and ~107 higher than that

of the more conventional visible/near-IR Raman scatter-

ing. Making the simplifying assumption that the number

of molecules probed via PD-LIF is fixed at n = 5 × 1013

molecules/pulse, we use our data at other fluences to esti-

mate the fluence dependence of σeff . This dependence is

compared to the Raman cross section, which remains fixed

with fluence, in Figure 7. As can be seen in the figure, PD-

LIF compares favorably with Raman scattering as a process

enabling standoff detection of TNT and other nitro-bearing

explosives. Using these estimates for the PD-LIF cross sec-

tion, we estimate that it will be feasible to achieve detection

of trace levels of explosives with a single pulse of eye-safe

UV illumination at distances of tens of meters.

Future WorkTo achieve these detection ranges (tens of meters) would

require ~30 cm optics and a laser capable of producing

at least 5 mJ pulses of light at 236.2 nm. While the laser

used in our laboratory measurements produces nearly

these pulse energies, it has several drawbacks. Its pulse

repetition rate is low (30 Hz), which makes rapid area

scanning impossible. It is also large and requires signifi-

cant input power. Moreover, it requires precise alignment

and as such is not robust enough for low-maintenance

field measurements. Finally, its bandwidth (0.03 nm) is

not well matched to the explosives signal peak (~0.5 nm;

see Figure 5).

John Zayhowski of Lincoln Laboratory’s Laser Tech-

nology and Applications Group has developed a design

for a solid-state laser that would meet all these specifi-

FIGURE 7. The TNT effective cross sections as a function of fluence for PD-LIF (squares) clearly outperform Raman-detection cross sections [14, 15], whose UV upper and infra-red lower bounds are denoted by dashed lines. Note that PD-LIF cross-section estimates assume that the number of particles probed per pulse was constant with fluence, and equal to the measured value at 10 mJ/cm2.

10 20 30Fluence (mJ/cm2)

Cro

ss se

ctio

n (c

m2 /m

olec

ule)

25

IR

UV

Raman

PD-LIF

155010–30

10–29

10–28

10–27

10–26

10–25

10–24

10–23

10–22

FIGURE 6. Dropcast TNT was analyzed to determine the trace-detection levels available with PD-LIF. On the left is a photo image of dropcast TNT on a silicon wafer with the laser spot (causing photolysis) labeled. On the right are time-series data displaying signal photons as a function of laser pulse (the top curve for TNT is offset for clarity from the bottom bare silicon).

00

5

10

TNT

Bare silicon wafer

10 20 30 40 50 60Pulse number

Sign

al a

t 226

nm

(arb

itrar

y un

its)

15

20

Laser ablation

1 cm


cal clutter, extensive studies are required to verify a low

false-alarm rate for a wide variety of substrates. Thus

our continued efforts are twofold: testing the new equip-

ment for standoff distance capabilities and evaluating

the specificity of the technique with other materials and

substrates to eliminate or at least significantly reduce the

false-alarm rate.

AcknowledgmentsWe thank John Zayhowski for his efforts in design-

ing the next-generation laser necessary to continue

this work in increasing the detection range. His

efforts stand out as a vital component for our future

research. This work was sponsored under the auspices

of the Lincoln Laboratory National Technology and

Industrial Base Program. n

cations. It would not only be more robust, smaller, and

lower power than the commercial system, but it would

also facilitate rapid area scanning and improve the sys-

tem’s signal-to-noise ratio (via a 0.5 nm laser bandwidth

matched to the explosives’ signal bandwidth). We have

identified the necessary crystal family and completed pre-

liminary fluorescence studies. These efforts should pro-

vide us with a stable high-power 236.2 nm laser source

capable of the desired high standoff distances. On the

basis of our examination of some nitrate-bearing materi-

als (fertilizer, soil, and manure), which showed no evi-

dence of a PD-LIF signal, we expect that there will be few

false alarms generated by other materials—i.e., we do not

expect nitrates to be a source of chemical clutter.

Although our initial work does not indicate the pres-

ence of significant sources of false alarms due to chemi-


REFERENCES

1. R. Kunz, J.D. Hybl, J.D. Pitts, M. Switkes, R. Herzig-Marx, F.L. Leibowitz, L. Williams, and D.R. Worsnop, private com-munication.

2. D.S. Moore, “Instrumentation for Trace Detection of High Explosives,” Rev. Sci. Instr., vol. 75, no. 8, 2004, pp. 2499–2512.

3. Kunz, private communication.4. Moore, “Instrumentation for Trace Detection.”5. T. Arusi-Parpar, D. Heflinger, and R. Lavi, “Photodissocia-

tion Followed by Laser-Induced Fluorescence at Atmo-spheric Pressure and 20° C: A Unique Scheme for Remote Detection of Explosives,” Appl. Opt., vol. 40, no. 36, 2001, pp. 6677–6681.

6. C.M. Wynn, S. Palmacci, R.R. Kunz, et al., “Experimental Demonstration of Remote Optical Detection of Trace Explo-sives,” Proc. SPIE, vol. 6954, 2008, 695407.

7. C.M. Wynn, S. Palmacci, R.R. Kunz, K. Clow, and M. Roth-schild, “Detection of Condensed-Phase Explosives via Laser-Induced Vaporization, Photodissociation and Resonant Excitation,” Appl. Opt., vol. 37, no. 31, 2008, pp. 5767-5776.

8. Wynn, “Experimental Demonstration.”9. Wynn, “Detection of Condensed-Phase Explosives.” 10. J. Luque and D.R. Crosley, “LIFBASE: Database and Spectral

Simulation Program,” SRI International Report MP 99-009 (1999).

11. J. Luque and D.R. Crosley, “Transition Probabilities and Electronic Transition Moments of the A 2Σ+–X 2Π and D 2Σ+–X 2Π Systems of Nitric Oxide,” J. Chem. Phys., vol. 111, no. 16, 1999, pp. 7405–7415.

12. L. Nagli and M. Gaft, “Raman Scattering Spectroscopy for Explosives Identification,” Proc. SPIE, vol. 6552, 2007, 65520Z.

13. L. Nagli, M. Gaft, Y. Fleger, and M. Rosenbluh, “Absolute Raman Cross-Section of Some Explosives: Trend to UV,” Opt. Mater., vol. 30, no. 11, 2008, pp. 1747–1754.

14. Nagli, “Raman Scattering Spectroscopy.”15. Nagli, “Absolute Raman Cross-Section.”

AbOUT THE AUTHORS

Charles M. Wynn is a technical staff member in the Submicrometer Technology Group, where he is working on laser-based trace detection techniques. He joined Lincoln Laboratory in 2000. He earned bachelor’s, master’s, and doctoral degrees in physics from the University of Connecti-cut, Carnegie Mellon University, and Clark University, respectively.

Stephen Palmacci is an assistant staff member in the Submicrometer Technology Group. He has worked for more than 35 years on laser-based research at Spectra Physics Laser Analytics division and Lin-coln Laboratory.

Roderick R. Kunz is a senior staff mem-ber in the Submicrometer Technology Group. His current focus is on the analysis and implementation of sensors for chemi-cal and explosives defense and threat signature phenomenology. He received his bachelor’s degree in chemistry from Rensselaer Polytechnic Institute and his doctoral degree in chemistry from the Uni-versity of North Carolina at Chapel Hill.

Mordechai Rothschild is the leader of the Submicrometer Technology Group. His interests are in standoff chemical sensing, microelectronics, optical lithography, and nanophotonics. He received a bachelor’s degree in physics from Bar-Ilan University in Israel and a doctoral degree in optics from the University of Rochester.

Documents

A Novel Method for Remotely Detecting Trace Explosives · A Novel Method for Remotely Detecting Trace Explosives Charles M. Wynn, Stephen Palmacci, Roderick R. Kunz, and Mordechai